A typical gas combusting furnace useful for space heating includes a furnace burner for generating heat, means for exchanging the heat from the burner to the heated space, and means for controlling the operation of the burner. The typical furnace burner of the premix type related to the subject invention includes a chamber wherein gas and air are mixed and then forced by a single speed, motor operated blower to a combustion chamber. In a typical premix type furnace burner, the mixture of gas and air is directed into the combustion chamber through a burner element such as a burner plate having a plurality of relatively small apertures.
In a typical gas combusting furnace utilizing a burner plate, the means for securing the burner plate commonly includes the use of screws or bolts, for example. In order to accommodate thermal expansion of the burner plate, it is often found necessary to leave the screws or bolts slightly loose, while providing enlarged or elongate holes in the burner plate for the screws or bolts to permit movement of the burner plate. When the burner plate is thus loosely secured, vibration of the burner plate and noise are often observed. It is also possible for a portion of the mixture of gas and air to enter the combustion chamber around the perimeter of the burner plate, thereby escaping combustion or being combusted outside the central flame zone at a relatively lower temperature, thus reducing the efficiency of the furnace.
A gasket may be provided adjacent the burner plate to attempt better control of the flow of gas and air, however, means for gasketing the burner plate generally prove to be prone to failure due to the temperatures encountered in combustion.
Some designs of burner plates seek to eliminate the undesirable flow of gas around the perimeter of the burner plate by providing a concave burner plate which, when thermally expanded, tends to increase in concavity and thus wedge against the securing screws or bolts. This solution is found to provide better control of combustible gas at the expense of increased stress and fatigue of the securing screws or blots and of the burner plate.
Another typical alternative design is to secure the burner plate in the furnace burner in a fixed manner, such as by welding, and thereby ignore the effects of thermal expansion. Designs of this type, while reducing objectionable vibration and noise, typically suffer from increased stress and fatigue of the furnace burner components, with concomitantly higher failure rates.
Previous designs of furnace burners have been typified by the requirement of a furnace burner uniquely sized to produce each furnace firing capacity. This is accomplished by the sizing of a blower and burner plate to achieve the desired firing capacity. The remaining components of the furnace burner must also be sized to accommodate the selected burner plate and blower. For each different firing rate, a furnace burner of the appropriate size, with all the appropriate components, is required. This results in undue multiplicity of components, and additionally makes difficult the stocking and servicing of furnace burners with correct components.
Typically, the furnace burner is operated intermittently when the furnace control means senses a requirement for additional heat in the heated space, based upon the tmeperature measured in that space. This heat requirement varies according to the season and climactic conditions, and the furnace operates as necessary to provide the heat required. The typical method of responding to these changes in heating demand is to change the duration of the operation of the furnace burner, or firings, and the interval between the firings. Thus, it is often found that the burner is operated for short perids of time during low heating requirement periods. This is inefficient and wasteful of fuel, since much of the heat generated is wasted in reheating the furnace components. An alternative method is to provide longer firings with increased intervals between the firings, which results in undesirable fluctuations between minimum and maximum temperatures found in the heated space.
Another alternative lies in reducing the gas input to the burner to provide a relatively lower heat output of the furnace in response to a lower heat requirement and thereby maintain the heat exchanger efficiency. However, changing the gas input rate without changing the corresponding air input rate changes the air to fuel ratio, causing improper combustion and substantially lowered efficiency of the furnace burner. Several patents have issued to address the problems inherent in typical premix furnace burners.
U.S. Pat. No. 4,465,546 discloses a variable rate burner having a throttle plate which is opened and closed to admit more or less air and gas for combustion. A multiple speed blower is not taught in the patent.
A variable rate gun type burner is disclosed in U.S. Pat. No. 4,424,793. The blower is driven by a single speed motor through a variable speed drive to supply air at a variable rate to the combustion chamber.
U.S. Pat. Nos. 4,334,885 and 4,340,355 teach the use of an induced draft apparatus having a variable speed blower in the exhaust of the combustion chamber. As in the foregoing patent, mixing of gas and air occurs at the point of combustion, and the blower acts only upon the air velocity through the furnace burner.
U.S. Pat. No. 4,480,629 discloses a burner with a plurality of small ports in a planar burner plate which is firmly secured to a header. No provision is made for thermal expansion or for interchangeability and standardization of the burner.
U.S. Pat. No. 4,397,631 teaches the use of an annular perforated burner which is domed to permit thermal expansion to cause co-axial movement of the burner. In the event of irregular circularity of the burner or housing, gas may bypass the burner causing incomplete combustion. This burner is relatively complex in design and does not permit interchangeability or standardization of furnace burner components.
A burner having a single port is disclosed in U.S. Pat. No. 2,162,084. A plenum with air admitting perforations is disposed adjacent to the burner. A retainer secures the burner assembly. No teaching or suggestion as to interchangeability or standardization of the burner components is found in the patent, nor does the teaching of the patent include any suggestion of modulation of heating capacity.
In consideration of the foregoing, it is an object of the present invention to provide modulation of the heating output in a premixing type furnace burner.
It is a further object to provide a furnace burner with superior means for directing and controlling the flow of combustible gas and air.
Another object of the invention is to provide a means for retaining a burner plate in a furnace burner while readily accommodating thermal expansion of the burner plate.
A still further object is to provide a furnace burner with components of a standard size.
Yet a still further object is to provide a furnace burner with a readily selectable firing capacity.
A still further object is to provide the foregoing in the simplest and most cost-effective manner.
These and other objects of the invention will be apparent from the attached drawings and the description of the preferred embodiment that follows hereinbelow.